The inventive subject matter relates to power conversion circuits and methods and, more particularly, to uninterruptible power supply (UPS) systems and methods.
UPS systems are commonly used in installations such as data centers, medical centers and industrial facilities. UPS systems may be used in such installations to provide backup power to maintain operation in event of failure of the primary utility supply. UPS systems may employ various topologies, including “standby”, “line interactive” and “on-line” configurations. In each of these, a battery or other auxiliary power source is used to provide backup power in the event of the degradation or failure of a primary power source, such as an AC utility line.
U.S. Pat. Nos. 6,160,722, 6,314,007 and 6,483,730, commonly assigned to the assignee of the present application, describe UPS systems that include a boost rectifier configured to be coupled to an AC source (e.g., a utility source) and a buck output inverter circuit configured to be coupled to the load and coupled to the output of the rectifier circuit by first and second DC busses. When the AC source meets certain criteria, the rectifier and inverter circuits may be operated to provide what may be described as a line-interactive (or line-adaptive) mode of operation, wherein the boost rectifier and/or the buck inverter may be used to provide amplitude control of the output AC waveform. When the AC source fails, an auxiliary power source (e.g., a battery) may be used to provide power to continue generation of an AC waveform at the load. This arrangement can allow for the use of relatively small capacitors coupled to the first and second DC busses, in contrast to the relatively large storage DC bus storage capacitors often used in “on-line” UPS systems.
The number of bus capacitors can be further reduced by using a switching circuit that alternatively couples the first and second DC busses to a neutral bus, such that a single capacitor may be used to support generation of positive and negative half-cycles of the output AC waveform. FIG. 1 illustrates a UPS system 100 with such an arrangement. The system 100 includes a boost rectifier circuit 110, a buck converter circuit 120, an inverter circuit 130 and a capacitor C1 coupled to first and second busses 105a, 105b. The boost rectifier circuit 110 includes a half bridge circuit including transistors Q1, Q2 that are configured to couple an input inductor L1 to respective ones of the first and second busses 105a, 105b. The buck converter circuit includes a half-bridge circuit including transistors Q3, Q4 that are configured to couple respective ones of the first and second busses 105a, 105b to an output inductor L2. The inverter circuit 130 includes a half-bridge circuit including transistors Q5, Q6 that are configured to alternately couple respective ones of the first and second busses 105a, 105b to a neutral N. A first relay circuit 150 is configured to selectively couple an AC source 10 or a battery 20 to the input inductor L1, and a second relay circuit 160 is configured to selectively couple the output inductor L2 or the AC source to a load 30, such that the power conversion chain may be bypassed by directly connecting the AC source 10 to the load 30. The transistors Q1, Q2, Q3, Q4, Q5, Q6 and the relay circuits 150, 160 are controlled by a control circuit 170 (e.g., a microprocessor or microcontroller and associated circuitry).
Under normal operating conditions when the AC source 10 is providing an acceptable voltage input, the AC source 10 is coupled to the input inductor L1 and the system 100 is operated in a line-interactive manner. When the AC source is within output regulation limits, it is passed directly to the output without boosting or bucking. When the AC input voltage becomes lower or higher then output regulation limits it high frequency boosts or bucks the AC input using the boost rectifier circuit 110 or the buck converter circuit 120. The inverter circuit 130 alternately connects the first and second busses 105a, 105b to the neutral N in successive half-cycles of the AC output to discharge the capacitor C1. In response to a failure of the AC source 10 such as voltage or frequency being out of specification, the battery 20 is connected to the input inductor L1. The boost rectifier circuit 110 may either be used to generate a fixed or time varying DC voltage across the first and second busses 105a, 105b from the battery 20, and the inverter circuit 130 and the buck converter circuit 120 may be used to generate an AC output at the load 30 from this DC voltage.